It is known to produce from such substances, transparent systems with light transmission characteristics which can be modified using an electrical signal. To this purpose, the electroactive substances are chosen depending on their redox potential, and on a light absorption and/or color variation between their oxidized and reduced forms. Thus, the light transmission of the system and/or its color can be modified by the electrical command.
The electrical command is an electrical potential difference or a current which is applied between two electrodes which contact a medium containing the electroactive substances. This medium is a liquid and/or a gel, so that the electroactive substances can each diffuse or migrate towards one of the electrodes. When a first one of the substances comes into contact with the electrode having the higher potential, it is oxidized. Simultaneously, a second substance coming into contact with the electrode having the lower potential is reduced. The two substances are therefore oppositely converted relative to the oxidized and reduced forms, and the overall variation in the light transmission and/or color of the system results from the individual variations in adsorption and/or color of each substance between its oxidized and reduced forms.
The two electrodes are transparent. They may be based on indium tin oxide (ITO), or based on tin oxide doped, for example, with fluorine (FTO for fluorine tin oxide), based on a sufficiently thin metal film, or on an invisible metallic mesh, etc.
The two electrodes may be placed on two opposite sides of a volume which encloses the medium with the electroactive substances. But in such a system configuration, a given light ray, transmitted through the system, passes through one of the electrodes, through the electroactive-substance-containing medium and then through the other electrode. The respective residual light absorptions of the two electrodes are therefore cumulative, causing a reduction in the light transmission of the system. Materials currently used for the electrodes have light absorptions that are perceptible, and which make the system too dark for certain applications.
According to a possible alternative electroactive-system configuration, the two electrodes may be juxtaposed on just one side of the medium containing the electroactive substances. But systems produced according to this alternative configuration are not completely transparent. They may scatter or diffract light due to the index difference between the material of the electrodes and the material present in the gaps separating these electrodes. In addition, if the distance separating the two electrodes is large, the gap separating these electrodes becomes visible due to the different light absorptions of the materials forming these electrodes and those forming the gap.
Conversely, if the distance separating the two electrodes is small, a strip may appear between the latter, in which the electroactive substances reciprocally neutralize one another. Such neutralization causes a progressive bleaching of the system. This is because the substances which have reacted on one of the two electrodes diffuse or migrate towards the other electrode, and react with the substances arising from the latter.